226 ABSORPTION OF KAIN AND DEW BY THE FOLIAGE-LEAVES 



the soil in wliicli the I'oots are imbedded that a gardener waters on dry days, 

 although incidentally he may pour the water over the aerial pai-ts of the plants. 

 He sees, however, that the water which falls in the form of rain or dew upon the 

 foliage and stems normally runs off them at once, or else collects in drops, which 

 trickle down whenever the plant is shaken by the wind, and are sucked up by the 

 thii-sty ground. This phenomenon must be due to tlie po.ssession by the leaves of 

 special contriv^ances to prevent their being wetted. It does not in any case support 

 the idea that foliage is as well adapted for the absorption of water as experience 

 has proved subterranean roots to be. This train of thought, which forces itself 

 upon every unbiassed observer of the processes as they take place in nature, is 

 certainly warranted in the majority of cases. Each absorption -cell on the roots 

 buried in the earth has an easily permeable membrane, and, as is well known, water 

 passes from damp earth through the cell-membranes into the interior of a plant 

 with great i-apidity. The water in the interior of the plant would be eijually easily 

 withdrawn through these cell-membranes by dry surroundings, but, as it is, this 

 scarcely ever happens, in consequence of the roots being situated underground. In 

 the case of aerial parts, especially the foliage-leaves, the circumstances ai-e quite 

 different. The leaves have to yield up to the air a portion at least of the water 

 conducted from the roots, because, as will be more thoroughly explained later on, it 

 is only by means of this evaporation that the entire machinery in the intei'ior of the 

 plant can be kept in motion. But this evaporation must not go too far; it must be 

 in proper relation to the absorption of water by the subterranean roots, and be 

 regulated to that end if the plant is not to run the risk of drying up altogether at 

 times — an occurrence which flowering plants are unable to survive, although the 

 mosses described in former pages have that power. Accordingly, in the case of the 

 foliage-leaves of flowering plants, evaporation is confined to certain cells and gi'oups 

 of cells, and these, in addition, have contrivances by means of which evaporation 

 can be entirely stopped on occasion of great drought. It stands to reason that all 

 contrivances which make it impossible for water to pass from the interior of the 

 leaves through the walls of the superficial cells into the surrounding air also hinder 

 the entrance of water into the leaves from the atmosphere. 



It would be altogether inconsistent with the system of arrangement of the sub- 

 ject adopted in this book if we were to discuss here all the contrivances serving to 

 regulate the exhalation of water by leaves, and we must, therefore, confine ourselves 

 to referring, by way of introduction, quite briefly, to the following facts, namely, 

 that those pores on the surface of leaves which are known by the name of stomata, 

 and are used as doors of egress by the exhaled water, do not admit rain or dew, or in 

 general, any water in the liquid state; that the so-called cuticle covering the exter- 

 nal walls of the epidermal cells in leaves is an additional barrier to both egress and 

 ingress of water; that when, in particular, this cuticle is furnished with a wax-like 

 coating, water does not adhere to the surface of cells so protected; and, lastly, that 

 atmospheric moisture can only penetrate into the interior of the plant at parts of 

 the leaves where the waxen incrustations are absent, where water i-emains adherent 



